The present invention relates to a method of producing trichlorosilane having a reduced content of dichlorosilane from a mixture of dichlorosilane and trichlorosilane. More specifically, it relates to a method of producing trichlorosilane having a reduced content of dichlorosilane by removing by-produced dichlorosilane efficiently for the production of high-purity polycrystal silicon.
The most common method of producing high-purity polycrystal silicon uses trichlorosilane as a raw material. Trichlorosilane is first purified by distillation and separated into high-purity purified trichlorosilane, other chlorosilane and impurities to be discarded. The "chlorosilane" referred to herein is a generic term for compounds comprising silicon, chlorine and/or hydrogen, such as trichlorosilane and silicone tetrachloride. Then, the purified trichlorosilane obtained by distillation is caused to react with hydrogen and the reaction product is subjected to thermal decomposition and reduction to produce high-purity polycrystal silicon. Particularly, a method of producing a rod-shaped deposit is called "Siemens method" and widely and commonly employed.
In a method of depositing polycrystal silicon using trichlorosilane as a raw material, dichlorosilane is by-produced and dichlorosilane is generally separated from trichlorosilane.
The separated dichlorosilane is discarded or recycled.
To recycle dichlorosilane, JP-A Hei 2-97415 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes a method of supplying dichlorosilane into a reactor for carrying out a reaction between silicon and hydrogen chloride. JP-A Hei 1-283817 proposes a method of obtaining trichlorosilane by reacting dichlorosilane with silicon tetrachloride in the presence of an amino compound catalyst on a resin. However, these methods require a large-scale apparatus.
To solve these problems and simplify the apparatus, JP-A Hei 7-315829 proposes a method in which an exhaust (vent) gas from various reactors in the production process of polysilicon is reacted with hydrogen chloride on a metal catalyst such as palladium or platinum at 30.degree. to 400.degree. C. to convert a Si--H bond contained in the exhaust gas into a Si--Cl bond. This method is excellent and can be carried out with simple equipment. It involves, however, a problem that due to its high reaction activity to chlorination, even trichlorosilane to be used for deposition is converted into silicone tetrachloride simultaneously when dichlorosilane is converted into trichlorosilane. Therefore, development of a method of allowing only dichlorosilane or a silane having a boiling point lower than that of dichlorosilane to selectively react with hydrogen chloride has been desired to overcome the above defect.